1. Field of the Invention
The present invention relates to a method of driving a semiconductor laser.
2. Description of the Related Art
A vertical-cavity surface-emitting laser (hereinafter abbreviated to VCSEL) is a laser which emits a laser beam vertically to a substrate.
The VCSEL laser has some advantages including the advantage of being able to be organized easily into a high-density, two-dimensional array, and a VCSEL array created by densely packing VCSEL lasers enables higher definition and faster speed when used for electrophotography.
In application to electrophotography, to form a latent image on a photosensitive drum, it is necessary to strictly control light quantity on a pulse by pulse basis.
To emit target light quantity accurately, the optical output power emitted from VCSEL is controlled by controlling driving current and drive time (pulse width) per pulse.
However, even if a driving current of a square waveform is applied, the VCSEL does not produce a square, stable optical output power, and consequently a delay occurs in the start of the optical output power emission.
To reduce such a startup delay, a technique described in Japanese Patent Application Laid-Open No. 2001-315381 improves the rise of an light waveform by generating a current waveform with a bias current not higher than a threshold value inserted in front of an image formation signal.
To improve the rise of the light waveform, the technique described in Japanese Patent Application Laid-Open No. 2001-315381 is configured to apply a current bias not higher than the threshold value for a predetermined period of time before each image formation pulse.
The technique described in Japanese Patent Application Laid-Open No. 2001-315381 improves the rise of the light waveform by applying a current bias not higher than the threshold value.
However, the VCSEL has a problem in that the optical output power cannot be controlled by the driving current alone because internal temperature of the VCSEL rises greatly during driving, device characteristics are sensitive to temperature, and the optical output power varies greatly with the temperature even if the same current is injected.
To describe further, in the application to electrophotography, there is a period of time during which the VCSEL is de-activated because the beam is located outside a photosensitive drum in addition to the period during which the VCSEL is being driven.
The de-activation period (a few tens of milliseconds or longer) is longer than a time constant (a few milliseconds) of VCSEL temperature changes.
Consequently, there occurs a phenomenon in which the temperature inside the VCSEL falls during the time interval between the end of driving for latent image formation and the start of the next latent image formation, and the internal temperature rises again at the start of driving.
The VCSEL undergoes greater rises in the internal temperature when emitting required optical output power (e.g., 1 mW) than does an edge-emitting laser. Furthermore, the VCSEL has the property of varying greatly in optical output power with the internal temperature.
Therefore, when the VCSEL is used as a laser source, the temperature inside the VCSEL changes from the start of latent image formation, causing a problem in that the optical output power varies from pulse to pulse even if the VCSEL is driven by a fixed value of current.
Also, in electrophotography, automatic power control (APC) cannot be used during light emission for latent image formation, and thus the variation in optical output with the internal temperature cannot be corrected using APC.
That is, the optical output power needs to be controlled solely by controlling the value of current used to drive the VCSEL. For that, an optical output waveform is required to have a short rise time upon injection of a fixed current and subsequently maintain a constant value.
For this purpose, with the VCSEL whose characteristics change greatly with temperature, the internal temperature of the device needs to be kept constant.
In view of the above problems, the present invention has an object to provide a method of driving a semiconductor laser, where the method can control changes in the internal temperature of a device as well as control optical output power using a driving current.